Pervaporation is a membrane-based process that can be used to remove a vaporizable component from a liquid mixture. In an example of such a process, water containing low concentrations of at least one organic compound is fed at essentially ambient pressure to the feed side of a membrane, while a vacuum pump or a gaseous sweep stream maintains a sufficiently low partial pressure of the organic compound on the permeate side of the membrane to provide a chemical potential gradient of the organic compound across the membrane. The organic compound and some of the water vaporize from the permeate side of the membrane to form a vapor-phase permeate.
One problem commonly associated with pervaporation is economically providing and maintaining the chemical potential gradient across the membrane. Those pervaporation processes employing a vacuum pump or condenser to provide the necessary chemical potential gradient are energy-intensive and thus expensive to operate. As the concentration of the organic compound in the feed stream is reduced to low levels, the partial pressure of the vaporizable organic compound in the permeate stream must be kept even lower for permeation and therefore separation to take place. If a vacuum pump is used to maintain the difference in partial pressure of the organic compound in equilibrium with he liquid feed stream and the partial pressure of the vaporizable organic compound in the vapor-phase permeate, the pump must maintain a very high vacuum, thus incurring high capital and operating costs. Similarly, if a condenser is used, extremely low temperatures must be maintained, requiring a costly and complicated refrigeration system.
A noncondensable sweep gas has been used to provide the driving force for the transport of vaporizable components across a pervaporation membrane. Although such a practice eliminates the vacuum pump-related problems associated with pervaporation, it introduces other problems associated with separating the condensable permeate from the noncondensable sweep gas. See, for example, Hoover et al., 10 J. Memb. Sci. 253 (1982).
Lee et al., in U.S. Pat. Nos. 4,933,198, 5,013,447 and 5,143,526, disclose a process for treating alcoholic beverages by pervaporation using a noncondensable sweep gas or vacuum. Although the sweep gas streams disclosed by Lee et al. may comprise water or ethanol, conditions in the sweep gas stream are controlled to ensure that these components are present only as noncondensable gas vapors and thus, no advantage from the use of a condensable sweep was recognized.
Another problem associated with pervaporation is providing the heat energy required for the vaporization of the permeate. In conventional processes the energy for this vaporization comes from sensible heat in the feed stream. However, as more and more permeate is removed from the feed, the temperature in the feed stream decreases. At such reduced temperatures, the partial pressure of the compound in equilibrium with the liquid feed is also reduced, reducing the driving force, thus causing the flux to decrease, which is undesirable. It is often necessary to employ a series of membranes separated by feed stream reheaters to maintain the temperature and increase the average flux. This practice results in increased costs and system complexity.
In 25 J. Memb. Sci. 25 (1985), Rautenbach et al. examined the use of a co-current condensable sweep as a source of heat. However, based on their calculations, the latent heat of the vapor carrier could be utilized only partially and, therefore, the use of a condensable vapor sweep in a pervaporation process would not be desirable.
Contrary to the currently accepted view, the present invention demonstrates that a pervaporation process utilizing a condensable vapor sweep is feasible.